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In this issue of Neurology®, Ryerson et al.1 report the results of a retrospective cohort analysis using the Food and Drug Administration–mandated TYSABRI Outreach: United Commitment to Health (TOUCH) database of all US JC virus (JCV) antibody–positive patients (≈35,521) receiving natalizumab infusions for multiple sclerosis (MS) treatment to evaluate the association between extended interval dosing (EID) compared to standard interval dosing (SID) and the risk of developing progressive multifocal leukoencephalopathy (PML). Three separate types of analyses were performed. In the primary analysis, the infusion frequency in the most recent 18 months was used to classify patients into SID and EID groups. In the secondary analysis, any period of EID >6 months was considered, and in the tertiary analysis, any patients who had received <10 doses per year of natalizumab were included in the EID group. For purposes of the primary analysis, the median average dosing interval in the patients on SID was 29.7 (interquartile range 28.6–31.4) days and in the patients on EID was 42.3 (interquartile range 39.0–49.1) days. The hazard ratios for developing PML in EID vs SID groups ranged from <0.01 to 0.12 across the analyses, corresponding to a remarkable risk reduction of 88% to 99%. This risk reduction corresponded to a drop from ≈4.16 to 4.74 PML cases per 1,000 in patients on SID to 0 to 2.04 cases per 1,000 in patients on EID. A popular aphorism variously credited to the astronomer Carl Sagan and the physicist and former Science editor Philip Abelson2 suggests, “Extraordinary claims require extraordinary evidence.” What are the limitations of this study, and what is needed to confirm these results?
Footnotes
Go to Neurology.org/N for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the article.
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- © 2019 American Academy of Neurology
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